1. Field of the Invention
The present invention relates to a projection display apparatus.
2. Description of the Related Art
When an image is projected by a projector (projection display apparatus) onto a screen in an oblique direction, the projected image is deformed into a trapezoidal shape. This phenomenon is called a trapezoid distortion or Keystone distortion. In order to correct the trapezoid distortion, a projector with the so-called digital Keystone correction function of changing the form of the projected image through digital image signal processing has already been commercialized. In a product having that function, a Keystone correction input amount is usually decided in a manual manner.
Also, an image projection apparatus is proposed which includes an inclination sensor in a projector body and automatically corrects the trapezoid distortion depending on the detected inclination angle (see Patent Document 1: Japanese Patent Laid-Open No. 2001-186538). For example, an acceleration sensor is employed as the inclination sensor.
FIGS. 7A and 7B show the principle for calculating the inclination angle by the acceleration sensor. The detection axis of the acceleration sensor is assumed to be parallel to an optical axis.
In FIG. 7A, reference numeral 10 denotes an acceleration sensor, and 100 denotes a projector. The projector 100 is horizontally installed in FIG. 7A. Therefore, a gravity component does not occur in the direction of the detection axis, and the output of the acceleration sensor 10 shows 0 [g] (g represents the acceleration of gravity).
On the other hand, in FIG. 7B, the projector is installed in a state inclined by θ [degree] relative to the horizontal. In this case, the acceleration sensor output shows sin θ [g]. Accordingly, the inclination angle can be detected by calculating arcsine (sin−1) of the acceleration sensor output.
FIG. 8 illustrates one example of the acceleration sensor output. The acceleration sensor output is detected in the form of an analog voltage output or a DUTY output. The number of detection axes ranges from one to three. Herein, a 1-axis analog voltage output is described as an example because it is relatively inexpensive to practice.
The acceleration sensor outputs a voltage in proportion to the detected acceleration (i.e., Vout_m1g [V] at −90 degrees, Vout—0g [V] at the horizontal, and Vout_p1g [V] at +90 degrees).
(Vout—0g-Vout_m1g) or (Vout_p1g-Vout—0g) is called “sensitivity” and represents an output voltage per 1 [g]. Also, Vout—0g is called “Og offset” and represents the output of the acceleration sensor at 0 [g] (=horizontal).
The acceleration sensor is constituted by a semiconductor in many cases, and “sensitivity” and “Og offset” have variations depending on individual parts.
Further, variations may occur in accuracy in mounting inclination of the acceleration sensor, accuracy in substrate mounting, etc. Therefore, acceleration sensors are desired to be adjusted individually.
In particular, a variation in the Og offset greatly affects the automatic trapezoid correction function that is often used near 0 degree of the inclination angle. Thus, projectors have to be individually adjusted in order to realize angle detection with high accuracy by using acceleration sensors.
Adjustment of individual acceleration sensors are generally performed by a method of experimentally measuring the relationship between the acceleration sensor output and angle with respect to a plurality of angles (see Patent Document 2: Japanese Patent Laid-Open No. 2004-334116), or a method of correcting values of “sensitivity” and “Og offset” of the acceleration sensor.
A flowchart of FIG. 9A shows the latter adjustment method. It is assumed that the detection axis of the acceleration sensor is arranged substantially parallel to the optical axis of a projection lens.
First, the adjustment is started in S101. In S102, the projector is installed in a vertically not-reversed state with the optical axis set in a horizontal state.
In S103, the acceleration sensor output (Vout—0) in the above state is stored in a storage unit. Namely:Og offset=Vout—0
In S104, the optical axis of the projector is set to an allowable maximum installation angle θm relative to the horizontal.
In S105, the acceleration sensor output (Vout_m) in the maximally inclined state is stored. Further, the sensitivity is calculated based on the following formula:sensitivity=(Vout—m−Vout—0)/sin θm 
In S106, the obtained Og offset and sensitivity are stored in a memory within the projector. In S107, the adjustment is brought to an end.
The relationship between the output of the acceleration sensor and the acceleration of gravity is linear. Accordingly, if the output of the acceleration sensor is Vout at an arbitrary inclination angle θ, the inclination angle θ is calculated by:
                    θ        =                ⁢                              sin                          -              1                                ⁡                      (                                          [                                  detected                  ⁢                                                                          ⁢                  voltage                  ⁢                                                                          ⁢                  at                  ⁢                                                                          ⁢                  horizontal                  ⁢                                                                                                    ⁢                                                                                                  ⁢                  reference                                ]                            /                              [                sensitivity                ]                                      )                                                  =                ⁢                              sin                          -              1                                ⁡                      (                          sin              ⁢                                                          ⁢              θ              ⁢                                                          ⁢              m              ×                                                (                                      Vout                    -                                          Vout_                      ⁢                      0                                                        )                                /                                  (                                      Vout_m                    -                                          Vout_                      ⁢                      0                                                        )                                                      )                              
The automatic trapezoid (distortion) correction can be accurately performed on the projected image by using the calculated angle.
In practice, however, when the projector is used in the vertically reversed state, it is sometimes used in a state hanging from the ceiling. When the projector is used in such a way, the trapezoid correction cannot be accurately performed in some cases just by using the above-described adjustment method. The reason will be described below with reference to FIGS. 10A-10D.
FIG. 10A shows the relationship between the optical axis of the projection lens and the detection axis of the acceleration sensor when the projector is horizontally installed in the vertically not-reversed state.
FIG. 10B shows the acceleration sensor output Vout_n0 in the above condition. In the illustrated example, the detection axis of the acceleration sensor is inclined by θG0 relative to the optical axis of the projection lens.
The acceleration sensor output Vout_n0 is adjusted to be equal to an output that is obtained when the projector is in an actual horizontal state, taking into account a variation in the “Og offset” output of the acceleration sensor and an error output caused by the mounting error.
FIG. 10C shows the relationship between the optical axis of the projection lens and the detection axis of the acceleration sensor when the projector is horizontally installed in the vertically reversed state after the above-described adjustment.
FIG. 10D shows the acceleration sensor output Vout_r0 in the above condition. Because the projector is horizontally installed in both the vertically not-reversed state and the vertically reversed state, Vout_n0=Vout_r0 should be resulted if the optical axis of the projection lens and the detection axis of the acceleration sensor are parallel to each other (i.e., θG0=0). In fact, however, an error of:ΔVg=sin(2×θG0)  (1)occurs as shown in FIG. 10D. That error is increased as the relationship between the optical axis of the projection lens and the detection axis of the acceleration sensor is deviated from parallelism in a larger amount.